The invention relates to substrate processing, and more particularly to transferring substrates to and from processing chambers where a substrate transfer shuttle may be maintained in a processing chamber during processing, such as during processing of solar panels.
Thick glass and metal substrates are being used for applications such as solar panels, among others. The thickness of such glass substrates may be, e.g., 6-9 mm. The processing of large glass and metal substrates often involves the performance of multiple sequential steps, including, for example, the performance of chemical vapor deposition (CVD) processes, physical vapor deposition (PVD) processes, or etch processes. Systems for processing such substrates can include one or more process chambers for performing those processes. For solar panels, CVD is commonly employed.
The substrates can have dimensions, for example, of 550 mm by 650 mm. The trend is toward even larger substrate sizes, such as 650 mm by 830 mm and larger. The larger sizes place even greater demands on the capabilities of the processing systems.
Some of the basic processing techniques for depositing thin films on large substrates are generally similar to those used, for example, in the processing of semiconductor wafers. Despite some of the similarities, however, a number of difficulties have been encountered in the processing of large substrates that cannot be overcome in a practical way and cost effectively by using techniques currently employed for semiconductor wafers and smaller glass substrates.
For example, efficient production line processing requires rapid movement of the substrates from one work station to another, and between vacuum environments and atmospheric environments. The large size and shape of the substrates makes it difficult to transfer them from one position in the processing system to another. As a result, cluster tools suitable for vacuum processing of semiconductor wafers and smaller glass substrates, such as substrates up to 550 mm by 650 mm, are not well suited for the similar processing of larger substrates, such as 650 mm by 830 mm and above. Moreover, cluster tools require a relatively large floor space.
Similarly, chamber configurations designed for the processing of relatively small semiconductor wafers are not particularly suited for the processing of these larger substrates. The chambers must include apertures of sufficient size to permit the large substrates to enter or exit the chamber. Moreover, processing substrates in the process chambers typically must be performed in a vacuum or under low pressure. Movement of substrates between processing chambers, thus, requires the use of valve mechanisms which are capable of closing the especially wide apertures to provide vacuum-tight seals and which also must minimize contamination.
Reducing the occurrence of defects in the substrate when it is transferred from one position to another is important. Similarly, misalignment of the substrate as it is transferred and positioned within the processing system can cause the process uniformity to be compromised to the extent that one edge of the substrate is electrically non-functional. If the misalignment is severe enough, it even may cause the substrate to strike structures and break inside the vacuum chamber.
Other problems associated with the processing of large substrates arise due to their unique thermal properties. For example, the relatively low thermal conductivity of glass makes it more difficult to heat or cool the substrate uniformly. In particular, thermal losses near the edges of any large-area, thin substrate tend to be greater than near the center of the substrate, resulting in a non-uniform temperature gradient across the substrate. The thermal properties of the substrate combined with its size, therefore, makes it more difficult to obtain uniform characteristics for the electronic components formed on different portions of the surface of a processed substrate. Moreover, heating or cooling glass substrates quickly and uniformly is more difficult as a consequence of its poor thermal conductivity, thereby reducing the ability of the system to achieve a high throughput.
In the case of solar panels, typical substrates are rigid panels of glass or a metal such as steel or aluminum. For front surface panels, metal is used. In this case, thermal losses are less significant. For back surface panels, either glass may be used. If the panels are glass, thicknesses of between 6 and 9 mm are often used. These types of substrates, for example, generally do not bow under the temperatures encountered in solar panel formation (e.g., about 300.degree. C.-400.degree. C.).
In certain types of solar panels, the active film is a PIN/PIN type or an NIP/NIP type, where the individual layers are made of doped or intrinsic amorphous silicon. In other words, a regular series of layers must be deposited on each substrate to form a solar panel. One drawback to current systems is that they generally do not allow for the sequential deposition of regular series of layers on consecutive substrates in a modular assembly-line like system.